1. Field of the Invention
The present invention relates to transmitting digital data over a band limited channel by using convolutional coding and multidimensional coded modulation.
2. Background Art
U.S. Pat. No. 4,077,021 to Csajka et al. used 2-dimensional coding to improve the performance of the modem against Gaussian noise. In this scheme the signal constellation was partitioned into 2-dimensional subsets. The minimum distance between the points in each subset was chosen to be larger than the minimum distance in the entire constellation. The indices of the subsets were found to be in one to one correspondence with the outputs of certain convolutional encoders. The input bits to the convolutional encoder were divided into two groups. During each baud (symbol interval), the first group of bits was expanded by the convolutional encoder to create subset selecting bits (coded bits) and the second group of bits, i.e., symbol-selecting bits (uncoded bits) was used to select a member of the subset to be transmitted. Although this coding scheme improved the performance of the system against noise, it also doubled the number of points in the signal constellation making the system more susceptible to other impairments. This is mitigated by the use of a multidimensional code, as described in European patent application No. 85300803.5 to Gallagher and U.S. Pat. No. 4,581,601.
The multidimensional encoder is usually a (m, m-1) convolutional encoder (i.e., the encoder produces m output bits for m-1 input bits) and when m is even, then m=2n, where n=2, 3, or 4 for 4, 6 or 8 dimensional coded modulation, respectively. Thus, a portion of the encoder output typically comprises a frame of n pairs of subset-selecting bits (i.e., bit pairs), with one of the bit pairs being indexed to a subset of signal points (symbols) of the constellation for each of n bauds. The symbol-selecting bits are used to designate one symbol from each subset of signals (symbols), as with the 2-dimensional convolutional coding. Thus, the encoding process thereby creates within a group interval n 2-dimensional symbols. The symbols within a group interval can be viewed as defining a 2n-dimensional point (symbol) from an available alphabet of 2n-dimensional symbols in 2n-dimensional space.
In a modem using multidimensional coded modulation, to obtain fractional bits per baud, the signal constellation can be divided into inner points and outer points, with one of the uncoded bits indicating for the n 2-dimensional (symbols) whether there are any outer points and at least one uncoded bit to indicate in which of the n bauds the outer point occurs, as shown in the article entitled "Efficient Modulation for Band limited Channels", G. David Forney et al., IEEE Journal On Selected Areas in Communication, Vol. SAE, 2 Sept., 1984, pp. 632-645.
To accomplish the decoding operations in the receiver for every received frame, the receivers must be properly synchronized with the incoming frames of bits, so that the receiver knows the beginning of each frame. During the beginning of the transmission a synchronizing sequence can be sent. However, whenever there is a loss of signal which is momentary this synchronization is lost. In such a case the receiver has to resynchronize without having to go through answer back sequence.
As is well-known, there are multiport modems which include a time division multiplexer for providing the user with the capability of transmitting more than one synchronous data stream over a single transmission line. A modem at one end of the line will have a plurality of input ports and the modem at the other end of the line will have a corresponding number of output ports. For example, assume a pair of modems with three ports A, B and C. The data transmitted by the data terminal at port A of the near end modem should be received by the data terminal at port A of the far end modem. Similarly, this is true for port B and port C. If the data transmitted from Port A at one end is received by port B or port C at the other end, then the ports are said to be "swapped". During the start up phase of the modem, a synchronization sequence is sent so that data reaches the correct port. This is referred to as multiport synchronization. However, whenever there is a temporary loss of signal for a few seconds and the signal is restored, the digital data should not be swapped. In other words, multiport synchronization should be restored. When the minimum port bit rate is equal to the signaling or baud rate or is an integral submultiple of the baud rate, multiport synchronization can be restored even with temporary loss of line signal. For example, the 16.8K b.p.s. modem with a baud rate of 2.4K bauds per second used in a three port configuration with port A at 4.8K b.p.s., port B at 2.4K b.p.s. and port C at 9.6K b.p.s. is grouped within each baud as: EQU Bit # 1 2 3 4 5 6 7 EQU Port A A B C C C C
All the information needed for synchronization is contained within each baud. Hence, if line signal is lost for a second or so, the hold over feature which retains baud clock information will prevent port swapping. On the other hand, applicants of the present invention, in designing a high speed modem were faced with the problem of having the baud rate not an integral multiple of the port rates, thereby causing the individual port rates to vary from frame to frame.